The transmission of digital information and data between systems has become an essential part of commonly used systems. With such systems, information content is transmitted and received in digital form as opposed to analog form. Information long associated with analog transmission techniques, for example, television, telephone, music, and other forms of audio and video, are now being transmitted and received in digital form. The digital form of the information allows signal processing techniques not practical with analog signals. In most applications, the user has no perception of the digital nature of the information being received.
Traditional modes of communication often occurred in "real time". For example, a telephone conversation occurred in real time. A "live" television sports broadcast occurred in real time. Users have come to expect these, and other such traditional forms of communication to be real time. Thus, digital transmission and reception techniques and systems need to provide for the real time transmission and reception of information.
There is a problem, however, in that digital communication between devices distant from each other usually precludes the availability of identical sampling frequencies. Except for those cases where a distinct clocking hierarchy structure can be defined and a common distributed clock source employed, there will be some difference between the sample rate of one device (e.g., the transmitter) from the other (e.g., the receiver).
Prior Art FIG. 1 shows a typical prior art digital information transmission and reception system 100. In system 100, a signal source 101, for example, a video camera, generates an analog input signal. The input signal is coupled to a sampler-ADC (analog to digital converter) 102, where it is sampled and encoded into a digital pulse code modulated signal. This signal is transmitted across a transmission link to a sampler 103. Sampler 103 is coupled to a DAC (digital to analog converter) reconstruction filter 104. The sampler 103 samples the pulse code modulated signal received via the transmission link. The sampling creates a digital signal, which is subsequently coupled to the DAC-reconstruction filter where it is decoded and filtered into an output signal. The output signal represents the input signal from signal source 101.
In system 100, as in other typical prior art digital transmission systems, the transmitter (sampler-ADC 102) and receiver (sampler 103 and DAC-reconstruction filter 104) operate at different sampling clock frequencies. Additionally, the ratio between the sampling clock frequency of sampler-ADC 102 and the sampling clock frequency of sampler 103 is a non-integer value. Hence, even though information is being transmitted at the same nominal sampling frequency, when the local clocks are not the same, there will usually be a slight difference in the actual sampling rates. As a result, sampling at the sending terminal (e.g., sampler-ADC 102) and reconstruction at the receiving terminal (e.g., DAC-reconstruction filter 104) is accomplished with a slight variance in the nominal sampling rates.
In addition, the sample rate frequencies may also vary over temperature, part scattering, and time. For clock ratios with a fractional part, a sample rate exists at which sample overruns or underruns at the receiver input will occur. Those overruns and underruns are hereafter referred to as sample slippage. Sample slippage generates objectionable distortion, for example, in the form of an audible click noise in audio transmissions and horizontal jitter in television systems. In some systems, the error due to slippage is cumulative and segments of transmitted information are backed-up and/or delayed. Over a period of time, such segments may eventually be periodically lost, especially if the system is designed to re-synchronize or to attempt to re-synchronize itself to real time or to a master clock.
Thus, what is required is a system for digital transmission which overcomes sample slippage limitations. The required system should provide for digital transmission and reception systems which eliminate sample slippage distortions. Additionally, the required system should function without requiring a large amount of computational power. The present invention provides a novel solution to the above requirements.